Cylindrical Langmuir probe measurements in a helium plasma were performed and analysed in the presence of a magnetic field. The plasma is generated in the ALINE device, a cylindrical vessel 1 m long and 30 cm in diameter using a direct coupled RF antenna (νRF = 25 MHz). The density and temperature are of the order of 1016 m−3 and 1.5 eV, respectively, for 1.2 Pa helium pressure and 200 W RF power. The axial magnetic field can be set from 0 up to 0.1 T, and the plasma diagnostic is a RF compensated Langmuir probe, which can be tilted with respect to the magnetic field lines. In the presence of a magnetic field, I(V) characteristics look like asymmetrical double probe ones (tanh-shape), which is due to the trapping of charged particles inside a flux tube connected to the probe on one side and to the wall on the other side. At low tilting angle, high magnetic field amplitude, power magnitude and low He pressure, which are the parameters scanned in our study, a bump can appear on the I(V) in the plasma potential range. We then compare different models for deducing plasma parameters from such unusual bumped curves. Finally, using a fluid model, the bump rising on the characteristics can be explained, assuming a density depletion in the flux tube, and emphasizing the role of the perpendicular transport of ions.

Potential and density structures in the vicinity of an radio frequency (RF) electrode/antenna in a magnetized plasma are investigated using an RF-compensated cylindrical Langmuir probe. These measurements were performed in the ALINE plasma device in which only electrons can be considered well magnetized. Very precise 2-D maps of the plasma parameters are drawn thanks to a 3-D automatic manipulator on which the probe is mounted. The effect of the tilted magnetic angle between the RF-biased surface and the magnetic lines is also studied thanks to a tilting electrode. Comparison of several simplistic models with the experiments proved the reliability of simple Langmuir probe measurements in such an RF and magnetized environment (space potential vs. tilting angle of the antenna with respect to magnetic field lines and recovery of the floating potential structure using measured currents). A fluid model based on total current density and ion diffusion equations over the biased flux tube provides the same density structures in front of the electrode as the measurements. Those density structures display a "bunny ears" shape and can be explained using transverse RF and collisional current behaviour: In front of the antenna, the transverse ion currents deplete the magnetized flux tube, while at the edge of the biased flux tube, the same currents increase the density.

The plasma-wall transition is studied by using 1d3V particle-in-cell simulations in the case of a one dimensional plasma bounded by two absorbing walls separated by 200 Debye lengths (lambda(d)). A constant and oblique magnetic field is applied to the system, with an amplitude such that r < lambda(d) < R, where r and R are the electron and ion Larmor radii, respectively. Collisions with neutrals are taken into account and modelled by an energy conservative operator, which randomly reorients ion and electron velocities. The plasma-wall transition (PWT) is shown to depend on both the angle of incidence of the magnetic field with respect to the wall, theta, and on the ion mean-free-path to Larmor radius ratio, lambda(ci)/R. In the very low collisionality regime (lambda(ci) >> R) and for a large angle of incidence, the PWT consists of the classical tri-layer structure (Debye sheath/Chodura sheath/pre-sheath) from the wall towards the center of the plasma. The drops of potential within different regions are well consistent with already published models. However, when sin theta <= R= lambda(ci) or with the ordering lambda(ci) < R, collisions cannot be neglected, leading to the disappearance of the Chodura sheath. In this case, a collisional model yields analytic expressions for the potential drop in the quasi-neutral region and explains, in qualitative and quantitative agreement with the simulation results, its reversal below a critical angle derived in this paper, a regime possibly met in the scrape-off layers of tokamaks. It is further shown that the potential drop in the Debye sheath slightly varies with the collisionality for lambda(ci) >> R. However, it tends to decrease with lambda(ci) in the high collisionality regime, until the Debye sheath finally vanishes.

The small layer oscillating in front of a radio frequency (rf) biased electrode in an asymmetric rf plasma discharge without a magnetic field is diagnosed using an rf compensated cylindrical probe. Thanks to this probe (0.15 mm in diameter), the floating potential is measured in this area. Radio frequency plasmas and sheath properties are then derived from the I-V characteristics measured by the probe at different rf power levels in both capacitive and direct couplings. In direct coupling, the plasma biasing is, as expected, nearly equal to the applied rf potential except at high power levels for which the current collected by the electrode saturates and the sheath potential gap is reversed. In capacitive coupling, the self-biasing of the electrode is strongly negative due to the matching box used. From the difference between the plasma potential and the floating potential, we found a sheath thickness of about 3 λDe. Within the rf power scan performed, the sheath thicknesses deduced from the potential and density profiles are 3 times higher than those from the Child–Langmuir law both in direct and capacitive coupling in a low collisional helium plasma.

In this work we report on the investigation of the transport behavior of Ti neutral atoms sputtered in a reactive high power impulse magnetron sputtering device used for TiN coating deposition. The time-resolved tunable diode laser induced fluorescence (TR-TDLIF), previously developed to study the transport of tungsten atoms, was improved to measure Ti neutral atom velocity distribution functions. We find that the TR-TDLIF signal has to be fitted using three Gaussian distributions, corresponding to the energetic, thermalized, and quasi-thermalized (atoms with non-zero mean velocity) atom populations. The ability to distinguish populations of atoms and to determine their corresponding deposited flux and energy may be of great interest to control film properties as desired for targeted applications. From the fitting, the vapor transport parameters (flux and energy) are calculated and studied as a function of distance from the target, pressure, and percentage of nitrogen in an Ar/N-2 gas mixture. The study focuses on the effect of added nitrogen on the transport of sputtered atoms.

Post-mortem analyses suggest that arcs in contact with metallic walls are the main cause of impurities and dust formation in tokamaks [1,2]. Such dust and impurities represent important operational and safety issues for next-step fusion devices like ITER and DEMO. If dipolar arcs, which occur in between adjacent Plasma Facing Components (PFCs), are well known and can in most of the cases be avoided by adapting the design of the PFCs, it is not the case for unipolar arcs taking place in between PFCs and the plasma. On addition to dust production, unipolar arcs also damage mirrors required by many diagnostics used for studying the plasmas as well as for ensuring a safe operation of the fusion device. Several models for explaining the triggering of unipolar arcs have been developed, but these models are based on assumptions which are difficult to assess experimentally, mainly due to the spatial and temporal scales involved (a few µm to <1mm and ~1ns for the triggering and few µs for the lifetime, respectively). As a result, defining the right threshold required for triggering the arcs is a tricky operation due to the uncertainties in the relative importance of the numerous input parameters which also influence the arc dynamics: surface roughness, secondary electron emission, thermionic emission, desorption of gas trapped in thick deposits, sheath modifications induced by ELMs… With the aim to provide more reliable input parameters and to assess the validity of the models, an experimental setup making possible highly resolved measurements in the linear low-beta ALINE plasma device has been developed. Stereoscopic fast camera measurements are performed and analyzed with the TRACK software, which enables a statistical investigation of the influence of various parameters on the triggering and dynamics of unipolar arcs.

Understanding and exploiting cylindrical Langmuir probe measurements in magnetized plasma is a real challenge, although this technique is the most common one used to access plasma parameters such as density, temperature, potential, etc. Since the magnetic field confines the electrons in the plasma, the measurement is actually done on the flux tube connected to the cylindrical probe. In some conditions, the I(V ) characteristics displays a bump between the exponential growth and the saturation of the electronic current. We propose a new interpretation of such distorted characteristics as a function of the angle between the probe and the magnetic field lines, the RF–power and the magnetic field magnitude. Introduction Cylindrical probe measurements give access to several plasma parameters such as temperature Te, density ne, potential φp, etc. In the absence of a magnetic field, the understanding of the I(V ) characteristics is straightforward [1]. But in numerous plasma devices there is a magnetic field to confine the plasma and to enhance the discharge. The presence of magnetic field can lead to misunderstanding and miscalculation of the plasma parameters as already pointed out elsewhere [2, 3]. Indeed the electrons are strongly "magnetized" since me mi⇔ ρce ρci, thus the way there are collected by the probe is affected. Several papers [4, 5, 6] have shown that under specific conditions (magnetic field amplitude, angle of the probe with respect to B, plasma density) a bump appears on the characteristics between the exponential part and the electron saturation current. The lower saturation of the electron current compared to the unmagnetized case is usually explained with an asymmetric double probe theory [7] or by the mean of an OML (Orbital Motion Limited) model [8]. But none of these models predict the raise of a bump in the I(V ) characteristics in an Helium plasma. In this paper we will study the dependence of the bump raise with ||B||, θ = (B, probe) and the input RF–power. We will also provide a simple fluid model to explain such characteristics.

A theory for data interpretation is presented for a cylindrical Langmuir probe in plasma parallel to the magnetic field direction. The theory is tested in a linear low-temperature plasma device Aline, in a capacitive radio-frequency (RF) discharge. The probe is placed on a 3D manipulator, and a position scan is performed. To exclude strong RF perturbations, the probe is RF compensated. Using the theory, electron densities are obtained from the current at the plasma potential, where no sheath is present. Results are calibrated by line-integrated density measurements of a 26.5 GHz microwave interferometer. Reasonable agreement is observed for probe and interferometer measurements. Furthermore, preceding, more general probe theory is compared to the one developed in the current work and the application limits are discussed.

Langmuir probe diagnostic on magnetic plasma devices often encounters more challenges in data processing than in non-magnetized plasmas, the latest itself being far from simple. In this paper, a theory of particle collection by a probe at the plasma potential in collisionless weakly ionized plasmas is constructed, accounting for velocities distributed according to the Maxwell equation and different mechanisms of particle collection depending on their speed. Experimental validation of the presented theory has been done with 2 cylindrical probes (rpr = 75 μm and Lpr = 1 cm and rpr = 0.5 mm and Lpr = 1 cm) parallel to B→ on a linear plasma device Aline, with magnetic fields of 0.0024–0.1 T and plasma densities of 1015–1017 m−3 in helium. Cylindrical probe measurements are compared to data from a planar probe perpendicular to the magnetic field, and the results for electron density, temperature, and plasma potential are presented. The introduced theory is initially constructed for a cylindrical probe but is applicable to various probe sizes, shapes, and orientations. Alongside the main subject, a number of associated issues are addressed with different details: a probe design issue relative to the magnetized environment, the “intersection” method of plasma potential evaluation, and the robustness of the conventional “1st derivative” method, a current bump near the plasma potential, lower limit for electron temperature estimation, and self-consistent calculation of electron temperature and density.

A quasi-static theoretical 1D model is developed to describe the sheath structure of a strongly emissive plasma-facing material and is subsequently applied to emissive probes' experimental data—which are usually supposed to be an efficient tool to directly measure plasma potential fluctuations. The model is derived following the space-charge limited emission current model developed in Takamura et al., [Contrib. Plasma Phys. 44(1–3), 126–137 (2004)], adding the contribution of secondary emission due to back-diffusion of plasma electrons at the emitting surface. From this theory, current-voltage characteristics of emissive probes are derived. A theoretical relation between the floating potential of an emissive probe and plasma parameters is obtained and a criterion is derived to determine the threshold between the thermoemission limited current regime and space-charge limited current regime. In the space-charge limited regime, a first order expansion is then applied to the quasi-static relation to study the effect of plasma fluctuations on emissive probe measurements. Both the mean values and the fluctuations of the floating potential of an emissive probe predicted by the model, as well as the potential value at which the transition between emission current regimes occurs, are compared to three sets of experimental data obtained in two different plasma devices.